First, save isn't necessary if the book is dirty, so don't... but that
means that the book has to be marked dirty after a session swap. No more
laziness.

Second, regardless of the outcome of inner_main_add_price_quotes the
session must be destroyed to remove the lock.

A couple of cleanups in QofSessionImpl::save as well: Rewrote the
descriptive comment to reflect how it really works when the backend has
gotten disconnected and removed the superfluous qof_book_set_backend
with the backend that we'd *just gotten from the book*.